This invention relates to a high pressure fitting for a closed hydraulic fluid system of the type used in a motor vehicle to actuate the braking system. More particularly, the present invention relates to a threaded connector for a braking system that maintains a fluid-tight seal if the threaded connector becomes loose after assembly and in operation.
High pressure hydraulic fluid systems are commonly used in motor vehicle braking systems. A supply of hydraulic braking fluid is maintained in a master cylinder and, upon actuation of the brake pedal, supplied at elevated pressure (i.e., 2000 psi) to individual wheel cylinders. The wheel cylinders, in turn, urge friction surfaces into contact to generate a braking force. Thus, several component interfaces exist in motor vehicle braking systems that must maintain a relatively high brake fluid hydraulic pressure throughout the service life of the braking system. In light of the importance of motor vehicle braking systems, any leakage of hydraulic brake fluid is unacceptable.
Threaded connectors are commonly used to attach brake tubing to the various braking system components in the manufacture of such braking systems. Such connectors typically include a nut having external threads situated over a flared tube end that is inserted into a mating threaded hole in the braking system component to which the tube is to be attached. An operator hand starts the nut into the hole, usually for two to three rotations. The nut is then secured by clockwise rotation of an air gun, which stops automatically when the proper torque has reached. When the gun stops, the operator withdraws the gun away from the component, preferably without imparting any counterclockwise rotation to the nut. The operator then opens the gun""s head and removes the gun.
The connector is sealed only if the clamping force generated by the interaction of the threads of the nut and hole is high enough to withstand the inner pressure, If the torque is too low, the clamping force will not compress the flared tube end sufficiently within the mating hole and the connector will leak. It has been found that even small counterclockwise rotation of the nut can dramatically reduce the clamping force and eventually compromise the seal. For example, if the nominal thread pitch is 1 mm, a one-quarter rotation obtains a 0.25 mm axial displacement. Since the tubing material is fairly compliant and is generally stressed beyond its yield strength during assembly, there is little residual elasticity in the system to accommodate even a relatively small 0.25 mm clearance appearing at the seal.
Thus, even slight counterclockwise nut rotation, such as that created accidentally by the operator from improper removal of the air gun after assembly, can be sufficient to compromise connector performance. This is particularly a problem arising from the time limitations for each line operation. In mass production vehicle assembly operations, operators typically perform 50 to 100 such fluid connection installations per hour. At such line speeds, the operator may be unaware of the inadequate connector torque condition or may be unable to correct the same if discovered. Thus, leaking caused by inadequate connector torque remains a chronic assembly line problem. Since very few leaks are caused by component damage, a solution of the problem of inadequate torque from improperly installed threaded tube connectors was sought to largely eliminate this assembly line problem.
One possible solution to the loss of clamping load due to counterclockwise rotation of the nut is to adopt a finer thread pitch to reduce the influence of counterclockwise rotation of the nut. A smaller thread pitch reduces the axial movement for a given amount of rotation. However, a finer thread pitch also tends to lower the thread strength and in turn lowers clamping load. Finer thread pitch also increases the possibility of cross-threading upon initial installation, the other major source of connector leakage.
Other solutions include the use a separate elastic element between the connector""s components, such as the flare, nut and hole, to assist the threads in compressing the flared tube end sufficiently into the mating hole. To a certain extent, such elastic elements do compensate for the reduction in the thread-generated clamping force caused by movement of the components. Several kinds of elastomeric materials, such as plastic or rubber, can be disposed between the connector""s elements to maintain a load on the connector. Likewise, coil springs can be provided about the brake tube for this purpose.
However, the main shortcoming of such solutions is that the load-deflection curve of such elastic elements is linear. Although linear elastic elements reduce the sensitivity to counterclockwise rotation and loss of clamping force, a linear elastic element will also suffer from a lower clamping force in proportion to the same displacement and to the same degree as that lost from the thread-generated clamping force at the same increment of rotation. Thus, the clamping force still depends on the relative position of the connector nut to the braking system component, since if counterclockwise revolution from a proper initial position occurs, the clamping force from the increasingly less displaced spring or elastic element will linearly drop until complete relaxation. For brake system tubing associated with high fluid pressure, the prior art failed to assure a desired clamping force within a realistic range of final connector torques and nut locations.
According to the present invention, a high pressure fitting for a closed hydraulic fluid system of the type used in a motor vehicle to actuate the braking system maintains a fluid-tight seal even if the threaded connector becomes loose after assembly and in operation. The connector sealingly mates an elongated tube to a component, where the tube has a flared, open end. The flared end forms an interior face and an exterior face and the component body has a recessed interior threaded hole axially aligned and in fluid communication with the flared open end of the tube. The threaded hole in the component body is countersunk to define a shoulder against which the interior face of the flared, open end of the tube is disposed.
The connector includes a nut concentrically disposed about the tube proximate the flared, open end of the tube. The nut has a threaded exterior portion and an abutting face proximate to and concentric with the exterior face of the flared, open end of the tube. A non-linear spring is disposed between the exterior face of the flared, open end of the tube and the abutting face of the nut. The non-linear spring is displaced upon tightening the nut to the body so as to generate a substantially constant restorative force over a portion of the displacement to urge the interior face of the flared, open end against the shoulder of the body. By careful selection of the non-linear spring""s physical properties to obtain a desired load-deflection curve, the clamping force can be maintained practically constant if deflection occasioned by a lowering in the connector torque and relative displacement of the connector nut occurs.
In the preferred embodiment, a Belleville spring provides the elastic element. Being turned out of its flat state, as it is caused to form a cone in the other direction after compression, a Belleville spring provides a practically constant force over a relatively wide range of deflection. Due to its relatively constant or flat load-deflection curve within predetermined range, the use of a Belleville spring in a brake tube connector maintains the same clamping force within a broader range of connector torque and nut position. For example, it has been found sufficient to employ a non-linear spring that has a constant load over a 1 mm deflection to provide a constant force over one full counterclockwise thread revolution. Thus, the brake tube connector of the present invention eliminates operator dependency and vastly reduces brake repairs at assembly plants.
The above brief description sets forth rather broadly the more important features of the present disclosure so that the detailed description that follows may be better understood and so that the present contributions to the art may be better appreciated. There are, of course, additional features of the disclosure that will be described hereinafter which will form the subject matter of the claims appended hereto.
In this respect, before explaining the preferred embodiment of the disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of the following description or drawings. The brake tube connector of the present disclosure is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for description and not limitation. Where specific dimensional material specifications have been included or omitted from the specification of the claims, or both, it is to be understood that the same are not to be incorporated into the appended claims.
As such, those skilled in the art will appreciate the conception, upon which this disclosure is based, may readily be used as a basis for designing other structures, methods and systems for carrying out the purposes of the present invention. It is important therefore that the claims are regarded as including such equivalent instructions as far as they do not depart from the spirit and scope of the present invention.
Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally, especially the scientists, engineers and practitioners in the art who are not familiar with the patent or legal phraseology, to learn quickly from a cursory inspection the nature and essence of the technical disclosure of the application. Accordingly, the Abstract is intended to define neither the invention nor the application, which is only measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.